1. Field of the Invention
This invention relates to a cold rolled steel sheet and a hot dip galvanized steel sheet which have a tensile strength (hereinafter abbreviated as T.S.) of more than 40 kgf/mm.sup.2 and are improved in ductility, particularly, in stretch flanging property, and processes for producing same.
2. Description of the Related Art
There has been an increasing demand for high tensile cold rolled steel sheets having a T.S. of more than 40 kgf/mm.sup.2, e.g., in automobile industry, to enhance the safety and reduce weight for fuel economy, as well as for hot dip galvanized steel sheets using a high tensile cold rolled sheet to improve the rustproof property. Further, there is a demand for construction materials having a smaller thickness to reduce the cost, and also in this field, high tensile cold rolled steel sheets are greatly demanded.
In these applications, high tensile steel sheets are required to have satisfactory workability, such as in pressing.
To meet these requirements, a process for producing high Mn--Si steel as a material has been proposed, e.g., in Japanese Patent Disclosure No. 57-63634 and No. 56-13437. In this process, however, an increased tensile strength is achieved chiefly by solution hardening, and therefore, a large quantity of Si, which serves to increase the strength, must be admixed, thus posing problems in surface properties and effectiveness of phosphatizing and hot dipping.
As a process which does not rely upon the alloy composition, unlike the above process, a process utilizing a annealed recovery structure is proposed, e.g., in Japanese Patent Disclosure No. 60-33318. However, this process has problems, such as fluctuation in properties, low ductility, and large planar anisotropy, and although the cost is low, the process is not efficient enough to permit a mass production.
This invention relates to a high tensile cold rolled steel sheet and a high tensile hot dip galvanized steel sheet which have a T.S. of more than 40 kgf/mm.sup.2 and solves the problems associated with the prior art, and an object thereof is to provide a high tensile cold rolled steel sheet and a high tensile hot dip galvanized steel sheet, both satisfying the below-mentioned conditions and having an excellent stretch flanging property, and processes for producing same.
(1) Eliminates the need for the admixture of Si which deteriorates the surface properties and the effectiveness of hot dipping, and provides a low alloy system.
(2) Improves the ductility, in particular, the stretch flanging property
(3) Achieves stable properties with less planar anisotropy.
(4) Imposes no restrictions on particularly severe operating conditions.